Flexographic printing processes use more and more so called "anilox" pitted cylinders for ensuring a suitable distribution of ink.
The advantage of a pitted cylinder arrangement comparatively to other arrangements that use a rolling of two smooth cylinders for pressure dosing an amount of ink, is a far better regularity of the film of ink that is distributed, whatever is the printing speed.
This is mainly due to the constant volume formed by the pits in the cylinder.
A first improvement of the distribution arrangement by pitted cylinder has been obtained by adding a doctor blade which rigorously removes the excess of ink whatever is the speed, and this is done better than with an ordinary drying cylinder.
A second improvement has been obtained by mounting a second doctor blade in order to form a seal tight feeding chamber, with a circulation of pressurized ink, which results in:
1--suppressing any overflow of ink and consequently making the flexo-machine clean; PA1 2--improving penetration of ink into the pits due to the pressure of ink within the chamber.
The known devices have however a defect comprising a printing of a phantom image.
The invention comes from the finding that the above mentioned problem can be explained as follows:
The pitted cylinder makes the ink contained in the pits to go out on the printing parts of the plate, thereby creating a negative image of the printed image.
The pits that have been drained of their ink become filled with air that enters the seal tight chamber.
This air must thus be expelled by the pressure of the ink circulating in the chamber and discharged with the excess of ink through the return opening.
Unfortunately it is found that the faster the cylinder rotates and the smaller the time for the pressure of ink to discharge the air contained in the pits, there is incomplete inking.
The ratio between the pitted arrangement and the plate carrying cylinder being generally different from 1:1, irregular printing results, including return of the image that has been previously printed. This drawback is known under the term of "ghosting", i.e. a phantom image.
Consequently, the purpose of the invention is to provide a better and faster discharge of air.
The prior art has disclosed an inking unit shown in U.S. Pat. No. 5,005,476 to Kobler and Ia. However this inking unit does not eleminate the above drawbacks.